clk: at91: pmc: export clock setup to pmc

Clock setup was intended for setting clocks at boot time on SAMA7G5,
e.g. for root clocks like PLLs, that were used to feed IPs needed alive
in u-boot (e.g. Ethernet clock feed by a PLL). Export this functionality
to all at91 clocks as it may be necessary on other SoCs.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>